This invention concerns a process for esterifying a carboxylate containing polymer, by reacting selected ammonium or sulfoxonium salts of the carboxylate groups to effect esterification.
Carboxylate containing polymers are often used in products that are in water solution, dispersion, or emulsion, where the carboxylate groups may help solublize or stabilize such mixtures. Such polymers are used for coatings, adhesives, photo and thermal resists. However, once the water is removed from the polymer, it is often desirable to somehow remove or convert the carboxylate groups into less polar moieties to reduce the effect of water on the resulting polymer (e.g., less water swell), or to help coalesce the polymer into a uniform film by removing highly polar regions made up of carboxylate salts. The removal or conversion of carboxylate groups should take place when desired, after most of the water has been removed, and the process should preferably be efficient and proceed under relatively mild conditions.
A. T. Lawson, et al., J. Chem. Soc., vol. 53, p. 624-636 (1888) and R. C. Fuson et al., J. Am. Chem. Soc., vol. 61, p. 1290 (1939), report that tetramethyl ammonium ions esterify certain carboxylate ions. K. Yamauchi, et al., J. Org. Chem., vol. 44, p. 638-639 (1979) report that trimethylsulfonium hydroxide esterifies carboxylic acids. Polymers are not mentioned in any of these papers.
U.S. Pat. Nos. 3,544,499, 3,804,797 and 3,903,056 describe the use of sulfonium salts of carboxyalate containing polymers for making such polymers less hydrophilic (more hydrophobic). No mention is made of the use of ammonium or sulfoxonium salts.
H. Ohtani, et al., J. High Res. Chromatog., vol. 14, p. 388-391 (1991) report the use of tetramethylammonium hydroxide as an alkylating agent to form methyl esters of the carboxylic acids residues present in liquid crystalline polyesters when such polyesters are analyzed by pyrolysis-gas chromatography. No mention is made of esterifying a polymer which contains carboxyl(ate) groups.